Immunoassays have been based on a variety of methods, including visual and radioactivity determinations. It also is known to optically evaluate immune reactions by using fluorescence intensity measurements. Although fluorescence intensity measurements are desirable in their simplicity, the usefulness of this technique is limited due to problems such as source fluctuations due to noise, drift and the like, fluorophore bleaching, and background fluorescence. Further, if the media is turbid or colored, the intensity measurements will be greatly affected. Moreover, since intensity is a linear product of numerous factors, such as the amount of fluorophore in each state, the excitation intensity, the excitation and emission bandpass, the wavelength sensitivity of the detector, and the like, a complex set of calibration curves must be used to correct for these factors. And finally, a change in the intensity of the probe does not necessarily occur upon binding of antigen and antibody. Thus, while fluorometric intensity measurements can provide useful and highly sensitive results under certain circumstances, they suffer from limited usefulness due to the problems outlined above.